General principles
If a piece of glassware is used to MEASURE volume of something, then it should be washed with the solution that is intented to be measured.
If a piece of glassware is used to hold a MEASURED AMOUNT of something, it should only be washed with distilled water NOT with anything else
The reason for the first principle is that you don't want distilled water remaining in the glassware to dilute the solution you want to measure, hence getting less of the 'substance' you want eg. acids/bases. The glasswares which fit in this category are pipettes and burettes
Following the second principle prevents the introduction of additional substances. Concentration here doesn't matter, because the amount of say acid molecues are fixed, dilution does not change how much acid(or base) the pipette has transfer.
so with HSC titrations, the flask should be washed by water